JPS6220510B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to scintillation cameras, commonly referred to as gamma cameras. The invention is particularly aimed at improving the uniformity and resolution of scintillation cameras.

In nuclear medicine, scintillation cameras are used to detect gamma ray photons emitted by bodies injected with radioactive isotopes. Photons are emitted corresponding to the extent to which the tissue under examination has absorbed the isotope. With proper processing, the signals corresponding to the photons can be used to generate a point-by-point image on a cathode ray oscilloscope corresponding to the emission pattern. The common camera device in use today is
It is based on the Anger camera described in issue 3011057. An Anger camera consists of an array of light-sensing devices, such as photomultiplier tubes, usually arranged in a hexagonal pattern, the input end of which is near a transparent plate or disk. Below the disk is a scintillation crystal that converts incoming gamma ray photons into photons of light or scintillation. A collimator is placed between the scintillation crystal and the body so that photons emitted from the body are incident perpendicularly onto the planar scintillation crystal.

Scintillation is detected by viewing the overlapping areas of the crystal where an array of individual photomultiplier tubes is used, using well-known electronic circuitry to convert the output of the photomultiplier tubes into x and y coordinate signals. This coordinate signal is used to control a cathode ray oscilloscope so that each point light source formed on the oscilloscope corresponds to a similarly located point on the crystal or body. The output signal is also used to generate the z signal,
This z signal turns on the cathode ray tube of the oscilloscope according to the calculated coordinates. The z signal is generated only when the energy of the scintillation event falls within the predetermined energy window. Photographic film can be used as an integrating device for the many light spots appearing on the screen of an oscilloscope.
A significant number of scintillation events are required to produce a final image of the distribution of radioactivity in the tissues of the body.

One problem with current scintillation camera equipment is that when a source of uniformly distributed radioactivity is placed near the crystal disk and an oscilloscope photograph is taken, non-uniformity appears in the photograph. , the bottom of each photomultiplier tube becomes a hot spot, and the space between the tubes becomes a cold spot. In other words, the scintillation event that actually occurred between the photomultiplier tubes is sensed as being partially transferred to the bottom of the tube, and the light spot density decreases between the tubes, and the light spot density decreases below the tube. Density increases in appearance. This phenomenon can be alleviated by moving the photomultiplier tube away from the disk, but
This reduces the camera's resolution of fine details. Therefore, to resolve fine details and maintain uniformity or correspondence between the generated image pattern and the displayed image pattern, the ordinary electrical signals coming into the device must be modified or corrected. There must be.

One method of making corrections using non-electronic means is described in US Pat. No. 3,774,032.
In this US patent, a mask is placed between the crystal and the photomultiplier tube to change the distribution of light sensed by the photomultiplier tube, so that the light from certain areas of the scintillation crystal is directly directed. Make sure it doesn't go to the photomultiplier tube. In this way, scintillation occurring directly below the multiplier will reduce the output of the multiplier, but in other areas,
That is, light from the area between the tubes can go directly to the multiplier tube. As a result, resolution is improved and image uniformity is improved.

In the past, it has been proposed to use electronic correction means to achieve results such as those obtained in this US patent. Without correction by electronic means or otherwise, the scintillation occurring in the area between the tubes will appear closer to the tubes due to inherent geometrical phenomena. This is called non-uniformity of displayed images. More specifically, the image obtained from a uniformly distributed isotope source will be denser or more concentrated beneath or immediately adjacent to the tubes than between the tubes. If the input and output signals of the preamplifier have a linear relationship, the imbalance between brightness and distance remains, except for low-level signals corresponding to noise, at or near the center of the tube. The rationale for electronic correction is that changing the output to suppress high-level signals corresponding to scintillation events will result in a more uniform distribution of light spots on the display. . Conventionally, by appropriately biasing the output of the preamplifier, high level signals can be clipped or suppressed; this effectively causes the preamplifier to It has been proposed and demonstrated that this would reduce the gain of

That is, the relationship between the input signal of the preamplifier and the output signal of the preamplifier is linear in a first signal range of relatively low levels, and after that point, it becomes linear for input signals of higher levels. lower the gain.

A device incorporating the single-break point concept has been built and tested and found to give better results than a linear amplification over the entire input signal range. However, the uniformity and resolution were still not optimal, as there were still some signs that the otherwise uniformly distributed light spots became non-uniform or concentrated. In other words, local hot spots
There are still spots and cold spots,
These appear randomly throughout the crystal and vary from device to device and depend on the individual characteristics of the device components.

Preferred embodiments of the invention provide that by having more than one change in the slope of the preamplifier's input-to-output transfer characteristic, hot and cold spots can still be present when using conventional methods of eliminating them. This is based on the recognition that it is possible to remove small localized hot spots and cold spots. That is, the present invention applies two or more selected bias voltages to the output of selected preamplifiers to optimize uniformity and resolution.

It will become clear how the advantages of the invention are achieved from the following description of preferred embodiments of the invention with reference to the drawings.

2 and 3 are simplified cross-sectional and side views of a scintillation camera in which the circuit of the present invention can be used. In FIG. 3, the scintillation camera is generally designated by the reference numeral 20. This is placed above the body 21. Assume that the body or its organs have absorbed a radioactive isotope, and one wishes to image the isotope and, therefore, the morphology of the tissue that has absorbed it. The isotope emits gamma ray photons and the gamma ray camera 20
receives it. The camera has a housing 22 that is opaque to radiation. A collimator 23 is coupled to the bottom of the housing. This collimator is
It consists of an array of tubes that are transparent to gamma-ray photons, with an opaque material between them. Inside the housing is a closed container 24 with a bottom 25 transparent to gamma photons. Immediately above the bottom 25 is a planar disk 26 made of a crystalline material that scintillates wherever it absorbs gamma ray photons, such as sodium iodide activated with thallium. An array of light sensing devices, such as photomultiplier tubes 1 to 19, is arranged above the scintillation crystal 26. A photomultiplier tube is coupled to the crystal 26 by a light pipe 27, which may consist of a glass plate.
Scintillation of crystal 26 is detected by these tubes, each tube producing a pulsed output signal for each scintillation event.

As seen in FIG. 2, this example uses an array of 19 photomultiplier tubes, arranged in a hexagonal configuration around a central tube 10. As is well known, the common number of tubes used in scintillation cameras is 19, but cameras with 37 photomultiplier tubes are also used. This invention is 19 or 37
or any other number of tubes.

FIG. 1 is a block diagram of the main components of a scintillation camera device in which the present invention may be utilized.
As shown, 19 photomultiplier tubes (inclusively indicated by the reference numeral 30 in this figure) work together to detect each scintillation, and their 19 outputs 31 are individually are separately coupled to a preamplifier circuit 32 of. The outputs 33 of the 19 preamplifier circuits are coupled to a resistor matrix and summing amplifier circuit 34 which outputs four coordinate output signals +x, -x, +y, -y from the outputs of the preamplifier circuits to lines 35 to 38. occurs in These four output signals are sent to a line amplifier and gated enlarger 39 and a z pulse shaper and pulse height analyzer (PHA) 40. A z pulse shaper combines the four input signals into a z signal corresponding to the energy of the scintillation event. z
The signal is passed through line 41 to differential amplifier/ratio circuit 42
is supplied to Pulse height analyzer 40 gates the gated enlarger when the energy of the scintillation event falls within the selected energy window, thus generating the stretched x, -x, + of lines 43-46.
y, -y signals can be supplied to the differential amplifier/ratio circuit 42. In circuit 42, the +x and -x signals and the +y and -y signals are subtracted, and z
The resulting ratio is taken using the pulse as the denominator to generate x and y coordinate signals on lines 47 and 48. When pulse height analyzer 40 determines that the scintillation event falls within the selected energy window, line 5
0, an erase stop signal is sent to the display cathode ray oscilloscope (CRO) 51, and the display CRO then places a light spot on its screen 52 at the calculated x and y coordinates. Occur.

The apparatus just described is essentially familiar as it pertains to the design and use of scintillation camera apparatus.

One channel is shown in FIG. 4 that generates a signal corresponding to the x and y coordinates of the scintillation detected by one photomultiplier. Certain portions of this circuit are well known in the art. Typically,
The channel has an input photomultiplier tube 30 of the well-known multi-dynode type. The anode of the photomultiplier tube 30 is connected to the resistor 5
3 to a high voltage source terminal 55 which is also connected to all other photomultiplier tubes in the 19 tube array 30, as shown by arrowed line 56. The output pulse signal from the photomultiplier tube 30 is fed to a non-inverting input of a signal conversion means constituted by an operational amplifier 57 acting as a preamplifier. The signal is coupled to preamplifier 57 from the midpoint 58 of a capacitive voltage divider made up of capacitors 59,60. The input of amplifier 57 is protected against excessive voltages by diode 61. A bias resistor 62 is also provided to set the offset to zero. The preamplifier has a feedback circuit composed of a resistor R2 and an input resistor R1, and a capacitor 6.
It is AC connected to the earth by 3.

As is well known, the amplitude of the pulse signal from the photomultiplier tube 30 is related to the distance between a particular scintillation event within the scintillation crystal 26 and the photomultiplier tube under consideration. Without the novel features of this invention, the input signal to amplifier 57 would be linearly amplified, resulting in a signal corresponding to the apparent distance of the scintillation event from the photomultiplier tube, as previously explained. However, this does not correspond to the true distance and leads to errors. As explained earlier, when directly aligned with a photomultiplier tube, even if the isotope source is uniform, there will be focusing effects that appear as hot spots in CRO displays. be. Even if a one-stage suppression scheme is used to suppress the highest amplitude signal, hot spots will still exist.

Regardless of whether the novel features of the inventive circuit described below are used, the output signal from preamplifier 57 is routed to resistor matrix 64 via resistor R3. This resistance matrix is used to calculate the x and y coordinates of the scintillation event that generated the pulse signal. A typical resistance matrix is shown in the figure. It consists of four voltage dividers. These voltage dividers are resistors +RX, R6, -
It consists of RX, R7, +RY, R8, -RY, and R9. As anyone who uses this type of matrix in scintillation camera equipment is well aware, the resistance in the RX and RY pair is
The weights represent or correspond to the inverse of the distance of a particular photomultiplier from the x or y axis of the photomultiplier array 30. As shown in FIG. 2, the y-axis passes through the center points of tubes 2, 10, 18,
The x-axis passes through the center points of tubes 8-12. The midpoint of the voltage divider is connected to common lines 35-38. A resistance matrix 64 with which each photomultiplier channel is associated.
A resistor having a specific value is connected to the common lines 35 to 38. For example, the end 65 of the common line is
It is connected to a resistive matrix associated with each other photomultiplier tube in array 30. The output signals on lines 35-38, representing the x and y coordinates of the scintillation, are applied to an amplifier and pulse stretcher, shown at block 39 in FIG. Thereafter, a light spot is generated at a corresponding position on the screen 52 of the CRO using the scintillation coordinates in a known manner.

We will now describe the means of the invention to make the amplification or output of the preamplifier 57 non-linear with respect to the pulsed output signal of the photomultiplier tube, which falls within two or more amplitude ranges. The lowest level signal is linearly amplified by amplifier 57 and sent unchanged to resistor matrix 64. A diode, not shown, can be inserted between the output of preamplifier 57 and R3 to block noise signals smaller than the forward threshold voltage of the diode. In this invention,
A bias voltage is selectively applied to an output line 66 connecting the output of preamplifier 57 to the resistor matrix. In this example, two biasing means 67, 68
is shown. The biasing means 67 comprises a diode D2 and a resistor R4 connected in series between the line 66 and the output terminal 69 of the bias or threshold voltage source 70.
Consists of. A wave capacitor 71 is connected between bias voltage source 70 and ground. The voltage at the output terminal 69 of the bias voltage source 70 is set to a threshold value that is slightly negative with respect to ground in this example. For this reason, diode D2 is normally reverse biased. The incoming pulse signal from photomultiplier tube 30 drives the output of preamplifier 57 in a negative direction. A negative bias applied through diode D2 or an output pulse signal below the threshold voltage is applied to resistor matrix 64 at its full amplitude by line 66. However, it limits the amplitude of signals appearing at the output of preamplifier 57 that are negative enough to exceed the threshold voltage of the bias source. That is, for signals that are more negative than the negative voltage applied through diode D2, the gain of the preamplifier is effectively reduced.

The other bias means 68 is R5, diode D
3, and a wave capacitor 72 is connected to the output terminal 73 of a second bias voltage source for setting a second threshold voltage. Source 74 is defined such that the bias voltage appearing at its output 73 is somewhat more negative than the bias voltage appearing at output terminal 69 of another voltage source 70. For this reason, diode D
3 is even more strongly reverse biased than diode D2. Thus, diode D3 is forward biased when the output signal from preamplifier 57 is somewhat more negative than is required to forward bias diode D2.

Bias voltage sources 70, 74 are similarly configured, but their outputs are set to different threshold voltage levels, as previously discussed. Batteries or other stable voltage sources can be used in place of sources 70, 74. In the illustrated example, source 74 is transistor 8
Consists of 0. The collector of this transistor is connected to the ground or to the midpoint of a dual power supply (not shown). A bias voltage is generated across emitter resistor 81. A constant base-emitter bias is provided through resistor 82 and is adjustable using potentiometer 83 to control the output voltage. The output bias voltage of the other source 70 is controlled by potentiometer 8.
4.

Those skilled in the art will appreciate that when the pulse output signal from preamplifier 57 goes positive, diodes D2 and D3 are connected to opposite polarities and bias sources 70 and 74 are
It is configured to supply a positive bias voltage to its output terminals 69 and 73 instead of a negative voltage as in this example.

FIG. 5 is a graph showing how the use of two biasing means 67, 68 affects the gain of the preamplifier. Diode D2
Until a first turning point 87, where the bias voltage applied to the output signal is less negative or more positive than the output signal, the optical gain increases, as can be seen from the first portion 86 of the transfer characteristic curve with a constant slope. The input signal from the multiplier which is in the first lower range is linearly amplified. Thus, an input signal such as shown at 88 within a first range up to the corner point 87 is linearly amplified, as shown by the corresponding output signal 89. The corner point at which the bias of diode D3 is overcome is shown at 90 in the graph. For this reason, the break point 87,
Any input voltage such as shown at 91 having a peak value within a second range between 90 and 90 will have an amplification degree, as can be seen by following the dashed line leading from the input signal 91 to the corresponding output signal 92. decreases, i.e. becomes non-linear. Corresponding to the corner point 90, when diode D3 is forward biased, the slope of the transfer characteristic, ie the effective gain of the preamplifier, changes again to a smaller value. Therefore, any input signal, such as shown at 93, having a peak value within the third range greater than the corner point 90 will be amplified even less, as can be seen from the corresponding output signal 94. Additional stepped bias voltages may also be used to create more than two breakpoints in the transfer characteristic, but good results were also obtained by using only two breakpoints. . A preamplifier 5 is used to eliminate response to low level noise.
Diode in series with the output of 7 (not shown)
, line 86 begins slightly off-axis from the zero axis in FIG.

The specific values of the bias voltages applied to diodes D2 and D3 are related to the characteristics of the particular circuit. However, in practical applications, the desired results were obtained by setting the lower bias voltage at output terminal 69 to approximately half the peak voltage at the output of preamplifier 57. The second bias voltage at terminal 73 was set to about 0.7 of the peak voltage of the signal. The peak voltage of the signal is determined by opening the series circuit of D2 and D3 and measuring the maximum signal obtained at the output of the preamplifier 57, or by biasing the diodes D2 and D3 to a value greater than the peak voltage. It can be measured by Thereafter, the diode circuit is closed and the voltage of the bias source is adjusted by the potentiometers 83, 84 until a uniform display is obtained on the display tube 52 when the radioisotope source is uniform.

The higher of the two bias voltages, i.e., the voltage from source 74, is only applied to the preamplifier output 57 for the group of photomultiplier tubes in the center in FIG.
i.e. tubes 10,5, in an array of 19 tubes.
It is applied only to the preamplifiers for 6, 11, 15, 14, and 9. The lower bias voltage provided by source 70 is applied to the outputs of all other tube preamplifiers. However, tubes 1, 3, 8, 1
2, 17, and 19, the value of R4 is the photomultiplier tube 2,
The value of R4, which enters the circuit of preamplifier 57 associated with circuits 4, 7, 13, and 18, is approximately 30% smaller.
For this reason, the pressure dividing effect performed by the action of voltage dividers R4 and R3 is slightly different between one group of tubes and another group of tubes. In the example, R4 and R5 for most tubes were 1 kOhm resistors, and for the preamplifier circuits associated with photomultiplier tubes 2, 4, 7, 13, and 18, R4 was given a value of 1.3 kOhm.

The field of view of the scintillation camera shown in FIG. 2 falls approximately within a circle slightly inside the center of the tubes 4, 13, 18, 16, 7, 2. Other tubes at the edge of the array have less signal contribution. Scintillation is only detected if it occurs inside the outer tube. This is the method of applying the bias voltage as described in the previous section. In any case, it is clear that the photomultiplier tube, which makes the largest contribution to the signal, has the strongest suppression. That is, hot spots are reduced and resolution is improved. Scintillation using more than 37 or 19 photomultiplier tubes
In the camera, the central group of tubes in the hexagonal array applies a stronger bias or threshold voltage to the output of its preamplifier 57, while the peripheral tubes in the annular configuration have a smaller bias. The above selective connection method is preferable, but even if all pipes are treated the same way,
Good results are obtained.

Although a specific method for creating multiple breakpoints in the transfer characteristics of a preamplifier for a photomultiplier tube has been described in detail, this explanation is only for illustrating the present invention, and does not limit the method for creating multiple breakpoints. Multiple contacts can be made in various ways.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a scintillation camera device that can utilize the present invention, Fig. 2 is a schematic diagram showing an arrangement of hexagonally arranged photomultiplier tubes in a scintillation camera, and Fig. 3 is a scintillation camera.・
4 is a schematic side view of the detection head of the camera; FIG. 4 is a circuit diagram of the circuit of the present invention; and FIG. This is a graph showing what happens when Explanation of main symbols 26: Scintillation crystal, 30: Photomultiplier tube, 32: Preliminary amplifier circuit, 3
4: Resistance matrix and summing amplifier circuit, 39: Line amplifier and gate enlarger, 40: Z pulse shaper and pulse height analyzer, 42: Differential amplifier and ratio circuit, 57: Operational amplifier, 64: Resistance matrix, 67 , 68: bias means, 70, 74: bias voltage source.

Claims (1)

[Scope of Claims] 1. An array of detectors 1 to 19 arranged to detect scintillation events caused by radiation emitted from a subject, each detector having a plurality of detectors 1 to 19 arranged to detect scintillation events caused by radiation emitted from a subject. A preamplifier 57 is provided for generating an electrical pulse signal having a magnitude dependent on the positional relationship of the detectors, the position of at least some of said detectors for each scintillation event being determined by said at least one detector. The output of the preamplifier is adapted to produce a non-linearity in the relationship between the electrical pulse signals generated by some of the detectors and the position coordinates of the scintillation event relative to the subject. In the scintillation camera of the type in which the output of the conversion circuit 34 to 50 is received and has a predetermined gain, a scintillation camera is attached to the output of the preamplifier to convert the electric pulse signal generated and the position coordinates. circuit means for linearizing the relationship between (a) a selected one of the preamplifiers, the electrical pulse signal having a first amplitude; range and within a second amplitude range, the transform is configured to suppress the pulse signal in order to improve the correspondence between the pulse signal and the position coordinates generated by the transform circuit. First circuit 6 that reduces the gain of the circuit
7, 70, and (b) the electrical pulse signal is coupled to the output of a selected one of the preamplifiers, and the electrical pulse signal is higher than the second amplitude range and within a third amplitude range; a second circuit 68 that further reduces the gain of the conversion circuit to suppress the pulse signal to further improve the correspondence between the pulse signal and the position coordinates generated by the conversion circuit; 74. A scintillation camera comprising: 2. The scintillation system of claim 1, wherein the first circuit and the second circuit have first and second branches 67, 68, respectively, having different conduction thresholds. camera. 3. The first branch 67 begins conducting at approximately 50% of the nominal signal value and the second branch 68 begins conducting at approximately 70% of the nominal signal value. Scintillation camera as described in Section 1. 4. A scintillation camera according to claim 2, wherein each of said first and second branches includes a biased diode D2, D3. 5. A scintillation camera as claimed in claim 4, wherein each of said diodes has a separate adjustable bias source 70,74 coupled thereto. 6 The first branch 67 and the second branch 68
4. A scintillation camera as claimed in claim 3, in which both are coupled only to the output of said preamplifier associated with a central group of detectors in said array.